I'm of the advice that lifetime licenses are only sustainable if they're sold as a way to attract early users, as a way to reward their faith into a product that had chances to fail and see their money gone forever. It shouldn't be a type of license that keeps getting sold once the product has matured and has some brand recognition in order to make it sustainable.

If the product can run standalone on the client's computer, make it a license for that specific major version, and sell the upgrade to the next major versions for a fraction of the price. Those who wants the latest features can buy the upgrade and sustain the business, and those who are happy with the current version can keep using their version without feeling wronged.